The present invention relates to a digital demodulation device for demodulating a signal whose phase is digitally modulated by, e.g., QPSK (Quadrature Phase Shift Keying) and, more particularly, to an improvement in an AFC (Automatic Frequency Control) circuit used for reproducing a carrier for detecting a phase of a digitally phase-modulated signal.
It is known that a transmission system for converting an information signal such as a video signal and a audio signal into a digitally phase-modulated signal using a digital phase-modulation technique such as QPSK and transmitting the signal, has recently been developed and used widely in various fields. On the reception side of the transmission system, the digitally phase-modulated signal is demodulated to reproduce the original video signal and audio signal.
A digital demodulation device is provided on the reception side of the transmission system, and reproduces a carrier in response to an input digitally phase-modulated signal and detects a phase of the digitally phase-modulated signal using the reproduced carrier. Such a digital demodulation device may include an AFC circuit for reproducing a carrier used in phase detection.
The technique of using the above AFC circuit is disclosed in European Patent Publication No. 0 373 405 A2, Japanese Patent Application KOKAI Publication No. 63-13552, Technical Report of IEICE (Institute of Electronics Information and Communication Engineers) AP97-14, SANE97-5 (1997-04), "Performance of a QPSK Demodulator using a Double-Product Type AFC Circuit for Very-Low CNR Environments," and the like.
The foregoing AFC circuit is constituted of an AFC loop including complex multiplying means for complex-multiplying a phase-detected signal by a carrier, phase detecting means for detecting a phase of a multiplication signal output from the complex multiplying means, frequency error detecting means for detecting a frequency error of a phase detection signal output from the phase detecting means, oscillating means for controlling an oscillation frequency based on the frequency error detected by the frequency error detecting means, and converting means for converting an oscillation signal output from the oscillating means into a carrier to be supplied to the complex multiplying means. In this AFC circuit, a frequency is pulled in order to eliminate frequency errors detected by the frequency error detecting means.
However, in the AFC circuit described above, a C/N (carrier to noise) ratio is lowered, as is the level of a frequency error signal output from the frequency error detecting means, thus causing a problem of requiring a long time for pulling a frequency in the AFC loop.